1. Field of the Invention
This invention relates to a method of development in a high density in electrophotography, and particularly to a development method of a two-component system directed to the prevention of toner scattering, the prevention of carrier drop off, the increasing of reproducibility of a dot image and the increasing of an image density.
2. Description of the Prior Art
A two-component type developer containing a magnetic carrier and a toner is widely used in commercial electrophotographic copying machines. In developing a charged image, a magnetic brush having this developer is formed on a development sleeve equipped with magnetic poles inside, and it is caused to slide with a photosensitive material having this charged image to form a toner image.
The requirement that in this two-component developer, a sufficient image density should be obtained without toner scattering and these characteristics should be maintained for a long period of time is determined by the affinity between the toner and the carrier.
As a general tendency, when the concentration of the toner becomes high, a high image density is obtained. But the triboelectricity of the toner tends to become insufficient, and the toner has a reduced ability to bind with the carrier. Furthermore, since the going and incoming of the toner in the developer become violent, the tendency of toner scattering is recognized to increase. For this reason, the conventional two-component development method is carried out at a lower toner concentration. But the development efficiency is generally low and the image density at the solid portion tends to be low.
If the charging properties of the toner and the carrier in the two-component developer are sufficient, it is expected that when the density of the toner is high, the image density is high and toner scattering becomes low. This is almost impossible of realization with a commercial toner or development method. In the toner production, poor toner particles containing no charge controlling agent or a little charge controlling agent will be formed necessarily with a certain probability, and during development, a charge controlling agent will be lost with a mechanical force in and outside a developer, or toner particles with a reduced content of the charge controlling agent will be formed with a certain frequency. Furthermore, since the area ratio of the document changes or the environment changes, the toner contains uncharged toner particles to which necessary charging cannot be given. Therefore, toner scattering necessarily takes place and this leads to the fouling of the inside of the copying machine or the fouling of the copied products.
The present inventors found that a developer comprising a two-component type developer comprising a magnetic carrier and a toner has a keypoint in the flowing state of the developer which passes a development area, and by setting development conditions within a certain range in regard to the flowing state, toner scattering can be effectively prevented even when the density of the toner is relatively high and the toner contains poorly charged particles. The inventors proposed it in Japanese Laid-Open Patent Publication No. 170953/1991.
The above proposal is generally satisfactory for the object of preventing toner scattering, and increasing the image density in a solid portion, but has still a problem in regard to the reproducibility of fine line images or dot images (digital images obtained by radar exposure) or carrier scattering.
In the above proposal, the developer is fed in a relatively sparse condition into a developing area. The image density in the solid portion increases but in the fine line portions or dot portions, images become thickened or toner scattering tends to occur. Furthermore, carrier scattering takes place, and the particle size or particle size distribution of the carrier tends to be not in accordance with actual operating conditions.